Third generation mobile systems, e.g. UMTS (Universal Mobile Telecommunications System), will provide not only circuit-switched services, but also packet-switched services similar to GPRS (General Packet Radio Service) designed for GSM (Global System for Mobile Communications). Packet-switched data transmission enables the use of different data services at a mobile terminal, while allocation of radio resources is required at the radio interface of the mobile system for each user, as necessary. When a user of a terminal in a UMTS system wants to use a packet-switched application, for instance to download a video or email from the network to the terminal device, a radio resource management system (RRM) of the UMTS system allocates an application-based capacity reservation to a radio bearer, which not only depends on the used application but also on the available radio bearer parameters. The radio bearer corresponds to the service provided by layer 2 of the transmission protocol for transfer of user data between the terminal device, e.g. mobile station (MS) or user equipment (UE), and the radio access network, e.g. UTRAN (UMTS Terrestrial Radio Access Network), GERAN (GSM/EDGE Radio Access Network) or IP RAN (IP based Radio Access Network). The radio interface corresponds to the interface between the terminal device and the access point of the RAN. This term encompasses all functionality required to maintain such interfaces.
In a typical one-way data transmission, for instance, when a file is downloaded from the network, a predetermined data rate can be allocated to a terminal in the downlink direction, i.e. from the base station to the terminal device. In such an application, in the uplink direction, i.e. from the terminal to the base station, data transmission bandwidth is typically low, consisting of, e.g., TCP (Transmission Control Protocol) layer acknowledgements.
One of the operations performed on the data transmitted over the radio bearer is the compression of the header fields of data packets. Header compression of transmitted data packets and decompression of received data packets is performed in a packet data conversions protocol (PDCP) layer of the UMTS system. The PDCP layer of the terminal device may support several header compression methods so as to enable connection set-up with as many layer protocol types as possible. Some header compression methods may also need a reverse connection for making different acknowledgements and solving error situations. More bandwidth then needs also to be reserved for the reverse connection, but, on the other hand, the compression of the header field decreases the need for a bandwidth in the forward direction of the connection.
Header compression of IP (Internet Protocol) flows is possible due to the fact that the fields in the headers of IP packets are either constant or changing in a known pattern between consecutive packets in the same flow. It is possible to send only information regarding the nature of the changing fields of the headers with respect to a reference packet in the same IP flow. The benefit is a significant reduction in header overhead and hence an increase in bandwidth efficiency. For example, IP based voice applications require an IP header of 20 octets for IPv4 and 40 octets for IPv6, UDP (User Datagram Protocol) header of 8 octets and RTP (Real time Transmission Protocol) header of 12 octets. When this is compared to the size of the payload which is of the order of 7 to 32 bytes, the gains from compressing the headers is quite apparent.
In order for header compression to work, there must be a compressor and a decompressor for each header compression context. During normal operation, the compressor will always try to send compressed headers instead of full headers. The compressed header represents the relative changes to the reference packet in the same header compression context and therefore the changes are relatively small. As IP based multimedia services are increasing rapidly, there is a need to support real-time IP services in the radio access network, e.g. UTRAN. However, with the added difficulties due to the radio interference there is a need for header compression to be robust in a cellular environment. Therefore, the IETF (Internet Engineering Task Force) has developed a Robust Header Compression (ROHC) scheme to standardize a header compression protocol suitable for wireless links. According to this ROHC scheme, the compressor starts in the lowest compressor state and gradually transitions to higher compression states. The general principle is that the compressor will always operate in the highest possible compression state, under the constraint that the compressor has sufficient confidence that the decompressor has the information necessary to decompress a compressed header. In the reliable mode, this confidence comes from the receipt of acknowledgements from the decompressor. Otherwise, this confidence comes from sending an information a certain number of times, utilizing a cyclic redundancy check (CRC) calculated over the uncompressed headers, and from not receiving negative acknowledgements. The compressor may also transition back to a lower compressions state when necessary. For IP/UDP/RTP, IP/UDP, ESP (Encapsulating Security Payload Header)/IP compression profiles, three compressor states have been defined, i.e. an Initialization/Refresh state (IR state), a First Order state (FO state), and a Second Order state (SO state). The purpose of the IR state is to set up or refresh the context between the compressor and the decompressor. The compressor enters this state at initialization, upon request from the decompressor or upon refresh time-out. The compressor leaves the IR state when it is confident that the decompressor has correctly received the refresh information. On the other hand, the compressor operates in the FO state when the header stream does not conform to a uniform pattern, i.e. constant changes, or when the compressor is not confident that the decompressor has acquired the parameters of the uniform pattern. The compressor will leave this state and transition to the SO state when the header conforms to a uniform pattern and when the compressor is sufficiently confident that previous non-uniform changes have reached the decompressor. Finally, in the SO state the compressor is sufficiently confident that the decompressor has also acquired the parameters of the uniform pattern. In the SO state the compressor sends headers, which mainly consist of a sequence number. While in the SO state the decompressor does a simple extrapolation based on information it knows about the pattern of change of the header field and the sequence number contained in the SO header in order to regenerate the uncompressed header. The compressor leaves this state to go back to the FO state if the header no longer conforms to the uniform pattern, or to the IR state if a counter so indicates in a unidirectional mode. Further details regarding the ROHC scheme can be gathered from the IETF specification RFC (Request For Comments) 3095 and from the 3GPP (Third Generation Partnership Project) specification TR (Technical Report) 25.844.
Currently, it is assumed that either the header compression is not taken into account when bearers are allocated, as for example in UTRAN Release 4 test specifications TS (Technical Specification) 34.108 and 34.123 parts 1-3, or the header compression is taken into account so that a lower bandwidth is allocated right from the beginning for a connection which uses header compression. In the former case, a problem arises due to the waste of resources, since the channel is reserved for full header and payload even though the actual bit rate of the compressed data is low. In the latter case, a problem arises in the beginning of the transmission when the header compression is started, as there is a need to send longer headers, i.e. full headers with some ROHC compression overhead to initialize the decompressor. If the channel is dimensioned for the narrow channels, some payload will be lost. Alternatively, a significant delay has to be allowed, which may not be possible due to real-time nature of RTP/UDP/IP traffic.
Another problem arises from the fact that the output of the compression cannot be predicted if it is allowed to use any header format. The headers can be encapsulated, they may have different options, either IPv6 or IPv4 may be used, the connection may be encrypted (ESP/IP), etc. Thus, the corresponding radio network controller device, e.g. the RNC of the UTRAN, should either know in advance the details of the headers of each specific connection, or the headers should be controlled by the network operator. The former case is not possible in the current specification releases. The latter case is possible in some specific cases, but its use will be very limited due to the fact that the network operators typically do not have control over the IP stack implementation in terminal devices, e.g. phones, laptops, PDAs etc., of the networks.